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Development of scanning electrochemical microscopy for the investigation of photocatalysis at semiconductor surfaces

Fonseca, Sofia Margarida Martins Costa da
(2002)
Development of scanning electrochemical microscopy for the investigation of photocatalysis at semiconductor surfaces.
PhD thesis, University of Warwick.

Abstract

This thesis is concerned with the development and application of scanningelectrochemical microscopy (SECM) to investigate interfacial photoelectrochemicalprocesses occurring at supported Ti02 surfaces. The new SECM approach, involvingboth amperometric and potentiometric electrodes, was used to monitor interfacialphotoprocesses with high spatial resolution.A new in situ photoelectrochemical approach to chemical actinometry has beendeveloped and used to determine the light flux through a quartz fibre employed in theSECM system. In this system an ultramicroelectrode (UME) probe is positioned withhigh precision at a known distance close to a TiOrcoated fibre and used to detectreactants or products of the ongoing photodegradation process. Themicroelectrochemical actinometry approach was developed using the well-known liquidphase potassium ferrioxalate actinometer. The approach involved recording the steadystatecurrent for Pe(III) reduction at an SECM tip positioned close to the fibre. A stepfunction in the light flux (off-on) was then applied and the resultingchronoamperometric behaviour due to the reduction of Fe(III) at the UME was recordedas a function of tip-fibre separation distance. A theoretical model has been developed tosimulate experimental current-time profiles, which allowed the light flux through thequartz fibre to be quantified.An experimental approach to investigating the photoelectrochemical reductionof oxygen at UV-illuminated Ti02 surfaces, in aqueous aerated and oxygenatedelectrolyte solutions, in the absence and presence of hole scavengers, has beendeveloped. In this new approach the chronoamperometric behaviour for oxygenreduction was recorded at an UME tip after stepping the light flux at a back-illuminatedTi02 film on and off The kinetics of the reduction process were interpreted throughvarious theoretical models proposed in the literature. This experimental approachdemonstrated a significant depletion of the oxygen concentration at the illuminated Ti02surface, which provides a new insight into the photomineralisation process, showing theimportant role of oxygen in controlling the kinetics.Using an SECM potentiometric approach, the photomineralisation kinetics of amodel organic pollutant, 4-chlorophenol (4-CP), in aerated and oxygenated aqueoussolutions at supported Ti02 films, were quantitatively investigated. A potentiometricAgi AgCI UMB, positioned at a known distance above the Ti02 film, was used tomonitor directly the cr production from the photomineralisation of 4-CP. A theoreticalmodel, employing a Langmuir-Hinshelwood type kinetic equation, has been developedto interpret the kinetics of the photomineralisation process and determine the associatedquantum efficiency. A direct correlation between oxygen consumption at theilluminated Ti02 surface and cr formation in the photomineralisation process has beenfound.SECM has also been used to monitor photoelectrochemical transfer kinetics atthe Ti02/aqueous interface using a well-known electron scavenger, methyl viologen.The Ti02 film was in contact with a solution containing methyl viologen dication(MV2+) as the redox mediator and sodium acetate as the hole scavenger. Thechronoamperometric behaviour for MV2+ reduction was recorded at an UME tip afterstepping the light flux at the Ti02 film from off to on. The rate constants for thereduction process were obtained through a theoretical model based on zero-orderkinetics. The substrate generation! tip collection mode of the SECM was also used todetect the MV.+ radical cation produced at the Ti02 surface at the UME tip.